A description will now be given regarding the coded_block_flag syntax element for image blocks using the 8×8 transform.
In the current design of the CAVLC 4:4:4 Intra, High 4:4:4 Intra, and High 4:4:4 Predictive profiles, a single coded_block_pattern is coded for each macroblock and the 4-bit variable CodedBlockPatternLuma is shared by all three color components when the separate_color_plane_flag syntax element is equal to zero. This works well except for Context Adaptive Binary Arithmetic Coding-coded (CABAC-coded) blocks using the 8×8 transform, that is, with the following parameters set as follows: the transform_size_8×8_flag syntax element is equal to one; and the entropy_coding_mode_flag syntax element is equal to one. With the current macroblock layer syntax, residual data syntax, and residual block CABAC syntax proposed for the High 4:4:4 Intra, CAVLC 4:4:4 Intra, and High 4:4:4 Predictive profiles, when the color components are encoded together in a macroblock, the coded_block_flag syntax element is also used if the transform_size_8×8_flag syntax element is equal to one.
With respect to code assignment for the coded_block_pattern syntax element in the 4:4:4 profiles, in particular, with the current semantics of the coded_block_pattern syntax element in the 4:4:4 profiles, only the 4 lower bits of the coded_block_pattern syntax element are used. Consequently, for the 4:4:4 profiles, part (b) (chrom_format_idc is equal to 0) of Table 9-4, the assignment of the codeNum syntax element to values of the coded_block_pattern syntax element for macroblock prediction modes, should be used.
Thus, the Context Adaptive Binary Arithmetic Coding (CABAC) disadvantageously fails when applied to block sizes of 8×8 in the High 4:4:4 Intra and High 4:4:4 Predictive profiles due to a deficiency of the coded_block_flag syntax element in such situations. Moreover, the Context Adaptive Variable Length Coding (CAVLC) process is inefficient for any block size with respect to the use of the coded_block_pattern syntax element in the CAVLC 4:4:4 Intra profile.